Magneto-optical recording discs wherein a dielectric layer and a magneto-optical recording layer are stacked on a transparent substrate are attracting much attention as large-capacity recording devices. Writing on these recording discs is effected by applying a magnetic field to the disc being locally heated by a laser beam to magnetize the disc in the desired direction. Reading of the recording disc is effected by irradiating the disc with a polarized laser beam to use Kerr effect or Faraday effect of the reflected light.
In these magneto-optical recording discs a dielectric layer is provided between the transparent substrate and the magneto-optical recording layer to enhance the magneto-optic effect. Materials normally used for the dielectric layer include oxides such as amorphous-yttrium-sialon (YSiAlON), SiO.sub.2, SiO, CeO.sub.2, ZrO.sub.2, TiO.sub.2, Bi.sub.2 O.sub.3 ; sulfides such as ZnS and Sb.sub.2 S.sub.3 ; and nitrides such as Si.sub.3 N.sub.4 and NbSiN. For instance, the U.S. Pat. No. 4,680,742 and the Japanese Provisional Patent Publication No. SHO-63-195,846 disclose a dielectric layer of amorphous-yttrium sialon. The Japanese Provisional Patent Publication No. SHO-57-169,996 discloses a dielectric layer of SiO. Further, the Japanese Provisional Patent Publication Nos. SHO-59-121,368, SHO-59-171,054 and SHO-62-172,545 disclose a dielectric layer of Si.sub.3 N.sub.4. Moreover, the Japanese Provisional Patent Publication No. SHO-59-171,055 discloses a dielectric layer of AlN. The Japanese Provisional Patent Publication No. SHO-61-258,353 discloses a dielectric layer of amorphous SiNx, and the Japanese Provisional Patent Publication No. SHO-63 -81,643 discloses a dielectric layer of NbSiN.
There is a problem. The substrate tends to warp after production since there is a large internal stress in the dielectric layer. When the substrate warps, it will become hard to track or focus on the magneto-electrical recording disc. The internal stress in the dielectric layer is related to the optical constants and the CN ratio of the magneto-optical recording disc, and can not be selected from the viewpoint of reduction in the angle of warpage alone. The film thickness of the dielectric layer has an optimal value for attaining the enhancement effect, and, therefore, can not be selected freely.